Microwave field effect transistors are used extensively in various types of radio frequency (RF) circuits, such as power amplifiers, RF switches, and other circuits. Various types of high-power microwave field effect transistors currently are produced, including aluminum gallium nitride/gallium nitride heterojunction field effect transistors (AlGaN/GaN HFET's), gallium arsenide pseudomorphic high electron mobility transistors (GaAs pHEMT's), gallium arsenide metal-semiconductor field effect transistors (GaAs MESFET's), and silicon laterally diffused metal-oxide semiconductor (Si-LDMOS) transistors.
A microwave field effect transistor used in an RF circuit may generate significant heat when non-zero voltage and current simultaneously appear at a semiconductor junction under operation. With an ever-present trend to increase the power handling capability of microwave field effect transistors, the junction temperatures of conventionally-designed transistors also continue to increase. In some cases, under high-power operation, junction temperatures may exceed the temperature rating of the device channel and present device reliability problems. It is clear that the temperature of the semiconductor junction under operation is a factor limiting the amount of RF power that can reliably pass through a transistor. Therefore, structures and methods that reduce operational junction temperatures are desired.